This invention relates to a transconductance amplifier having a variable transconductance, said transconductance amplifier comprising a non-inverting and an inverting input terminal, an output terminal and a control input for controlling said variable transconductance, said input terminals being coupled to respective inputs of a differential stage, said differential stage having a first differential output coupled to said output terminal via a first output branch of a first current mirror and a second differential output coupled to said output terminal via a second current mirror and a first output branch of a third current mirror.
The invention also relates to a variable gain stage comprising an operational amplifier having a non-inverting and an inverting input terminal and an output terminal and a gain control input, said gain stage comprising a resistive ladder coupled between said output terminal and a reference terminal, said inverting input terminal being selectively connectable to one of a plurality of taps of said resistive ladder in response to a binary gain control signal applied to said gain control input for digitally controlling the gain of said variable gain stage.
The invention further relates to an automatic gain control circuit comprising a variable gain stage coupled to a peak detector, an output signal of said peak detector being applied to a gain control input of said variable gain stage.
A transconductance amplifier as described in the opening paragraph is known from IEEE Journal of Solid-State circuits, Vol. SC-17, No. 3, June 1982, page 522. The known transconductance amplifier comprises a differential stage having a first and a second output branch, each of these branches carrying a common-mode current and a differential current proportional to the difference between the voltages on the input terminals. The two current branches only differ in that the differential current in the first branch is opposite in sign to the differential current in the second branch. By subtracting the currents at the output terminal only the summed differential currents will appear at this output terminal. By changing the tail current of the differential stage the transconductance of the transconductance amplifier is made variable using an analogue control signal.
In mixed analogue and digital applications it is, however, desirable to be able to change the transconductance in response to a binary control signal. This usually involves using a D/A converter for converting the binary control signal into an analogue control signal. A drawback of this method is that a D/A converter is needed.